Safe storage of flammable, corrosive and toxic liquids (hereinafter "dangerous liquids") has become a topic of vital importance to an environmentally concerned public and to the government in the face of increasingly frequent reports of contamination from leaking storage tanks and toxic spills.
Underground storage tanks have been widely used for storage of dangerous liquids across a broad range of many industries. This widespread prior use appears to be premised upon a rationale that such tanks are safer. The earth is frequently viewed as providing a natural containment vessel for the tank, and actually provides some protection against interference from the surface environment and from natural and manmade occurrences and activities.
Single walled underground storage tanks for gasoline and other petroleum products were typically formed of coated steel. As such tanks aged and/or corroded, or were exposed to vibration or seismic occurrences, susceptibility to leakage increased. Small leaks were not originally given much concern. However, with increasing concern for the environment and detection of pollution of ground water from leaking tanks, public concern has increased. In modern times, older tanks are routinely excavated and replaced at considerable cost and with down time of the activity making use of the storage tank being replaced.
One known advantage of an above-ground storage tank for dangerous liquids and the like is that it may be readily inspected visually for any leaks, an advantage not available to underground tanks. However, much greater care must be given to the design and construction of above-ground tanks to be sure that susceptibility to leakage is minimized. Double walled tanks have been proposed to solve leakage problems. In a double walled tank, an inner tank provides a primary containment vessel for the dangerous liquid. An outer, continuous tank provides secondary containment against leakage and also provides an insulation barrier against ambient external interferences. It was also feasible to place sensors in the space between the walls of the outer and inner tanks to monitor for leaks. As experience with double walled tanks grew, it became apparent that such tanks also were susceptible to leakage.
Leakage was found to be caused by cracks or tank ruptures produced by a number of factors, such as unequal weight distribution of the stored liquids, unequal stress distribution throughout the tanks because of construction methods, seismic and thermal forces acting upon the tanks, weathering and corrosion, and combinations of the above listed factors.
Various construction materials and methods have been employed to solve the problem of leakage in double walled storage tanks. Such materials and methods are frequently required to meet government regulations regarding the storage of flammable materials; and, Underwriters Laboratory, Inc. approval is also frequently required by governmental regulatory agencies.
Reinforced concrete is commonly employed in the construction of double walled storage tanks of various shapes and configurations. Typical prior art methods of construction involved forming an outer tank or containment vessel of concrete by pouring or placing plastic-phase concrete around the inner tank which acted as a form. Steel reinforcement material was placed outside of the inner tank and typically connected thereto. The concrete was then applied to complete the double walled tank. A method for forming a double walled storage tank for dangerous liquids out of preformed modules is described in U.S. Pat. No. 3,404,500 to Akita et al. It has proved difficult to ensure that the reinforced concrete, which is vulnerable to temperature changes, will be crack resistant. Concrete absorbs heat which causes expansion of the stored liquid within the tank and thereby increases stress upon the tank. In addition, construction techniques may produce air pockets in the concrete and/or the insulation substance used between the tanks; and, it is difficult to assure dissemination of the concrete to all recesses of the space between the double tanks. In addition to such distribution and curing problems, double walled tanks preformed of reinforced concrete are very heavy and cumbersome to transport and install at the use site. The increased weight also contributes to internal stresses within the tank structure and may lead to cracking and failures of the concrete outer containment tank.
Contemporary above-ground double walled tanks for storing dangerous liquids are typically rectangular. These tanks are constructed with steel inner tanks encased in six-inch thick steel reinforced concrete outer tanks. Such tank structures weigh approximately 18,000 pounds and are subject to considerable internal stress forces at various locations, such as the junction of the base and the side walls. Various methods are used to decrease leakage in such tanks, such as coating the inner tank with fluid-tight barrier substances, using a compartmentalized inner tank which attempts to confine leakage to the affected compartment of the inner tank, using cylindrical tanks, wrapping the inner tank with plastic materials to provide an additional barrier, and reinforcing the areas subject to the most stress.
Two prior art double tank construction methods are discussed in the Kotcharian U.S. Pat. No. 4,513,550 and the Lindquist et al. U.S. Pat. No. 4,826,644. The Kotoharian method is drawn to a rectangular tank constructed by building an outer pre-stressed concrete tank, coating the inner walls of the outer tank with a fluid-barrier substance, mounting a pre-coated steel inner tank within the outer tank, and filling the space between the two tanks with stacked blocks of polystyrene insulation in order to accommodate the severe thermal gradient arising from storage of liquid energy gas at extremely low temperature.
The Lindquist method calls for construction of a rectangular storage tank by gluing polystyrene panels to the outer surface of an inner steel tank. A polystyrene liner is then placed between a form defining the contour of an outer tank and the polystyrene panels, and the outer tank is formed of plastic-phase concrete emplaced between the form and the inner tank.
Additional insulating materials have been used in tanks for transporting hazardous materials and for storing cryogenic materials. The Gablin et al. U.S. Pat. No. 4,100,860 discusses a hinged or otherwise openable transportation tank with an outer shell constructed from a ductile metal. Polyurethane foam is used as insulation to permit safe transportation at maximum temperatures of approximately 100 degrees Fahrenheit. The Gerhard U.S. Pat. No. 4,098,426 also discusses the use of polyurethane foam material for insulation in transportation tanks. Polyurethane foam insulation does not provide sufficient fire protection for tanks used to store flammable liquids.
The Newman, Jr. et al. U.S. Pat. No. 3,942,331 discusses a cryogenic tank having a deformable inner tank and synthetic, resinous cellular insulation to maintain the very low temperature of the storage material. A conduit is emplaced in the insulation to maintain a sub-atmospheric pressure between the inner and the outer square tank.